I own a Casio solar (brand name "Tough Solar") watch, which is generally the same technology IIUC. I believe they indeed have long life, but there is actually still something of a "battery" inside (specifically, I think a kind of a capacitor, though not 100% sure), which still has some life expectancy and a number of cycles it can survive. Not to mention that even this kind of a watch has a number of potential other failure modes as well... just recently I stumbled and dropped it, and the back-plate sprung away. Surprisingly, even a watchmaker took a while to put it back in, and was similarly mildly amused that what looked like a trivial job proved to not be exactly so. That said, it was more of a suit watch, totally not a G-Shock.
Some data: I had a "Tough Solar" G shock for field work- it was very long lasting. I got it in 2002 (if memory serves) and it lasted about a dozen years. By that point the capacitor could not hold a charge very long at all, and it became unusable. I set it aside and a few year later the rubber bits fell apart including stuff around the case. I'd say by 15y it was totally toast. Not bad considering I never did a single bit of service on it, but definitely a reminder that time always wins in the end. Even when it comes to your timepiece!
G-Shock basically solved the what-time-is-it problem when they got ruggedness, solar power, and remote time signal update into a cheap package. Expensive watches that don't have those features are jewelry.
